Spark gaps adapted to generate an arc between the electrodes, and with a careful time determination, are utilized, inter alia, in high-voltage laboratories for triggering laser beams and as protection for series capacitors in electric power lines. The present invention is primarily intended for applications within the latter field but is not in any way limited thereto. Series capacitors are used in electric power lines, primarily for increasing the transmission capability of a power line. Such series capacitor equipment comprises a capacitor bank that is connected to the power line and is traversed by the current of the power line. The voltage across such a series capacitor becomes proportional to the current in the power line, and in case of an over current in the power line, for example caused by a short circuit in the power network, an overvoltage arises across the series capacitor. It is previously known, for the purpose of protecting the capacitor from such overvoltage, to connect the capacitor in parallel with a spark gap that is triggered in a suitable manner in case of an overvoltage across the capacitor. In this way, the line current is shunted past the capacitor, which in this way is protected. Known protection devices of this kind are described, for example, in U.S. Pat. No. 3,725,729, U.S. Pat. No. 4,625,254, U.S. Pat. No. 4,652,963, U.S. Pat. No. 4,703,385, U.S. Pat. No. 4,860,156, U.S. Pat. No. 5,325,259, U.S. Pat. No. 5,893,985, U.S. Pat. No. 6,700,091, U.S. 2008/253,040, U.S. 2009/134,129 and USH756.
One disadvantage of conventional ignition of the arc in the main spark gap based on an auxiliary spark gap, that is where the main spark gap is triggered to ignite via a spark generated by a triggering circuit, is that it requires a very high voltage across the main spark gap. The reason for this is that the mode of operation is based on the auxiliary spark gap substantially serving to ionize the air between the main electrodes. The ionization facilitates the formation of an arc between these; however, it assumes that the voltage is sufficient for a flashover to arise. The voltage across the main spark gap must amount to at least some 10 kV. This limits the possibilities of application. Further, it requires reconditioning of the spark gap even after a few discharges because the corrosion caused by the arc on the electrodes results in the electrode distance being influenced, which, in the case of such a conventional kind of spark-gap triggering, influences the tripping level, that is, at which voltage across the main spark gap that an arc is formed.
The above described disadvantages have to a large extent been overcome by the device disclosed in WO 03/096502. In the device of that disclosure each auxiliary electrode is provided with guide rails designed such that the arc, via the guide rails and under the influence of the generated inherent magnetic field, moves into the main electrode gap, each of the two guide rails having a length that is larger than the width of the auxiliary spark gap, and which auxiliary electrodes are arranged such that they are protected from the effect of plasma formed in the main spark gap and whereby a hermetic enclosure encloses the main spark gap and the auxiliary spark gap.
The generation of the arc in the main spark gap is achieved with that device in a way that is fundamentally physically different from what is achieved with conventional technique. With conventional technique, the arc in the main spark gap is achieved by an igniting spark from the auxiliary spark gap ionizing the air between the main electrodes so that a flashover arises therebetween, which presupposes a very high voltage therebetween. With the special design of the auxiliary spark gap according to the mentioned disclosure, the generation of the arc in the main spark gap is not correspondingly dependent on such ionization. The guide rails result in the arc in the auxiliary spark gap, by the inherent magnetic forces that arose around the arc, being brought to successfully move inwards towards the main spark gap so that gradually the arc is established between the electrodes of the main spark gap.
An important consequence of this difference is that no bias voltage is needed across the main spark gap in addition to the arc voltage drop and the electrode voltage drop. It may therefore be sufficient here with a voltage of the order of magnitude of 1 kV or even lower.
The fact that no high voltage is required across the main spark gap entails considerable advantages. The function of the spark gap will be relatively insensitive to the variation of its width. In this way, the spark gap need not be reconditioned after a discharge. The spark gap may thus be activated hundreds of times without any requirement for intermediate service. Further, the spark gap may be used for new functions where no high voltage arises when the spark gap is to be activated. Further, the spark gap is insensitive to the external environment, such as moisture, ice, snow, dirt and insects. Since the auxiliary electrodes are protected from the effect of plasma formed in the main spark gap, the risk that the arc in the main spark gap may damage the auxiliary electrodes is avoided.